Electrolyte for and method of polishing metal surfaces anodically



Patented May 1942 ELECTROLYTE FOR-AND ME'rnon or POL- ISHING MET-AL-SURFACES ANonIcALnY Charles L'; Faust, Columbus, Ohio, assignor toBattelle Memorial Institute; Golumbus, Ohio, a corporation of Ohio NoDrawing. Application August 18,1939, Serial No. 290,787

Claims.

This invention relates to'an electrolyte for and to a method offinishing metal surfaces anodically. More particularly the inventionrelates to the anodic treatment of metals in an aqueous electrolytecontaining sulphuric acid and arsenic acid to impart to the surface ofthe metal a lustrous, mirror-like polish.

It has heretofore been proposed to use various compositions ofelectrolyte for the anodic treatment of metals. In some cases the objecthas been to clean the metal surface by the etching action of theelectrolyte, while in other cases the etching effect has been reduced toa minimum and the object has been to produce a lustrous, imagereflective surface. The anodic treatment-of aluminum has a somewhatseparate status, since there the purpose is not only to provide alustrous surface but also to form thereon a dense, adherent layer ofaluminum oxide as a protection therefor against corrosion and wear.

I have now found that an electrolyte comprising sulphuric and arsenicacids can be satisfactorily employed in the anodic treatment of variousmetals and alloys to impart to the surfaces thereof the desired type offinish. In the preferred embodiment of m invention, which will bedescribed more specifically hereinafter, such an electrolyte is employedfor imparting a lustrous, mirror-like surface, but its use is notlimited to that purpose alone.

It is therefore an important object of my invention to provide anelectrolytic solution for use in the anodic treatment of metalsgenerally to impart thereto a desirable surface finish.

It is a further important object of my invention to provide anelectrolytic solution for use in the anodic treatment of metalsgenerally to impart thereto a desirable surface finish.

It is a further important object of my invention to provide anelectrolyte for and a method of anodically treating metals and alloys toproduce thereon surfaces that are highly lustrous and free from thescratches and piled layers characteristic of mechanically polished metalsurfaces.

Other and further important objects of this invention will becomeapparent from the following description and appended claims.

A method to which this invention pertains involves making the metals tobe treated an anode in an electrolytic bath of suitable composition andpassing a current therethrough of suflicient density and for asuflicient length of time to produce the desired type of surface finish,which, more specifically, may be a high lustrous or mirror-like polish.

The electrolyte that I have found to possess the most generalapplicability comprises a mixture of sulphuric and arsenic acids, orsulphuric acid and a compound furnishing arsenate radicals in solution.For satisfactory results, the combined acid concentration should be atleast 50% by weight of the solution, but if it is desired to obtain thehighest degree of polish, the minimum combined acid concentration shouldbe not less than by weight. In general, the sulphuric acid should bepresent within the proportions of from 5 to 60% and the arsenic acidwithin the proportions of from 5 to the balance being largely water; Allproportions are expressed as percentages by weight.

While it is possible to produce desirable surface effects with mixturesof. sulphuric and arsenic acids containing as much as 50% water, Iprefer to keep the water content of the electrolyte relatively low,since I have found, in general, that baths containing the lowerproportions of water may be operated at lower current densities, andtherefore at a lower cost for electrical current. On the other hand, itis desirable to have some water present, the preferred minimum of watercontent being about 10%, although electrolytes containing as little as5% of water will be found to work with success.

It will be understood, of course, that during the operation of anelectrolyte such as described, there will be a gradual building up ofthe content of metal salts formed by dissolution of the metal undergoinganodic treatment. There may also be some change in the water content dueto loss of water by evaporation or electrolytic decomposition, or due togain of water by condensation or collection of moisture from the air.The composition of the electrolyte should, however, be kept within theforegoing limits for good operation.

The following will serve to illustrate preferred compositions of myelectrolyte for use in the electropolishing of stainless steel, nickeland nickel alloys:

Percentages by weight Bath No. 1 No. 2 No. 3

Percent Percent Percent Sulphuric acid 36 25 1O Arsenic acid (HsAsOl) 4769 77 Water 17 6 13 In general, however, anode current densities varyingfrom 50 to 1,000 amperes per square foot may be employed, depending uponthe type of surface treatment that is desired. Higher current densitiesthan indicated by the foregoing range may also be employed, but in thatcase the higher current cost offsets the saving in time. The length oftime to effect the desired results depends upon the magnitude of thecurrent density employed, and to some extent upon the particular metalsundergoing treatment and the character 'of their surfaces initially.Rough surfaces, of course, require a longer time to polish thanrelatively smooth ones.

As illustrative of the length of time required to effect a high degreeof electropolish, 20 to 90 minutes is usually found suflicient. Thetemperature at which the anodic treatment is carried out, whilepreferably between 50 and 75 C., may be varied from around roomtemperature to temperatures only slightly underthe boiling point ofwater.

From the foregoing description of my invention it will be apparent thatI have provided a novel composition of electrolyte for use in the anodictreatment of metals. s

It will, of course, be understood that various details of the processmay be varied through a wide range without departing from the principlesof this invention and it is, therefore, not the purpose to limit thepatent granted hereon otherwise than necessitated by the scope of theappended claims.

I claim as my invention:

1. An electrolyte for use in the anodic polishing of metals, comprisingan aqueous solution of sulphuric acid and ortho-arsenic acid, thecombined acid concentration being at least 50% by weight and the balancebeing largely water.

2. An electrolyte for use in the anodic polishing of metals, comprisingan aqueous solution of sulphuric and arsenic acids, the sulphuric acidbeing present in an amount between 5 and 60% by weight and the arsenicacid being present in an amount between 5 and 90%, the combined acidconcentration being at least 75% by weight and the balance being largelywater.

3. An electrolyte for use in the anodic polishing of metals, comprisingan aqueous solution of about 36% sulphuric acid, 47% arsenic acid and17% water by weight.

4. An electrolyte for usein'the anodic polishing of metals, comprisingan aqueous solution or about sulphuric acid, 69% arsenic acid and 6%water by weight.

5. An electrolyte for use in the anodic polishing of metals, comprisingan aqueous solution of about 10% sulphuric acid, 77% arsenic acid and13% water by weight.

6. The method of anodlcally polishing metals, which comprises making ametal article the anode in an aqueous electrolyte of sulfuric andarsenic acids the combined acid concentration being at least and thebalance being largely water and passing an electric current therethroughof suflicient density and for a suflicient length of time to effect apolish.

7. The method of electropolishing metals,

I which comprises making a metal the anode in an electrolyte comprisingfrom 5 to of sulphuric acid, from 5 to 90% of arsenic acid, the combinedacid concentration being at least and the balance being largely water,and passing a current therethrough of sufficient density and for asumcient length of time to effect a polish on said metal surface.

8. The method of electropolishing metals, which comprises making a metalthe anode in an electrolyte comprising about 36% of sulphuric acid, andabout 47% .of arsenic acid, the balance being largely water, and passinga current therethrough of sufficient density and for a suflicient lengthof time to effect a polish on said metal surface.

9. The method of electropolishing metals, which comprises making a metalthe anode in an electrolyte comprising about 25% of sulphuric acid, andabout 69% of arsenic acid, the balance being largely water, and passinga current therethrough of sufficient density and for a suflicient lengthof time to effect a polish on said metal surface.

10. The method of electropolishing metals, which comprises making ametal the anode in anelectrolyte comprising about 10% of sulphuric acid,and about 77 of arsenic acid, the balance being largely water, andpassing a current therethrough of suflicient density and for asuflicient length of time to effect a polish on said metal surface.

CHARLES L. FAUST.

